How far is that joist, at the right, from the cut out. Looks rather close.
You could sister one or two 2X's to it with screws and PL Premium.

 

Just got home from work!
The measurements look like...
The subfloor over-hang I have left is 1-3/4" (maybe a little more)
The "under wall" hidden from view distance is about 2-1/4"

 

Put 2x4 on the flat 1/2 under the plywood and screw them there.

 

Put 2x4 on the flat 1/2 under the plywood and screw them there.
View attachment 665745
View attachment 665746

Wow! That fix looks very doable!
Should I install any vertical blocking into the joist cavities, too?
And here I was, thinking I may need to use perpendicular joist hangers with 2x6's between the outside wall (on the right) and the nearest joist

 

You should check to see if the copper pipe is touching the toilet drain.

 

Good call!
I did some under-floor investigating and snapped as many photos as I could!

The closest copper pipes that are touching anything, seem to be fastened under the joists just below, and, directly on the side of the joist on the left.
I imagine that If I wrestled too much with the drain, that also has a section going through that joist, I could damage that copper pipe.
I will be careful.
Does there look to be anything else concerning the pipes?

Plumbing on the Right side
Plumbing under the Center area
Plumbing on the Left side